The present invention relates to nuclear magnetic resonance imaging systems for providing images of distributions of a chosen quantity in a selected region of a body. The examination may be of many different kinds of bodies but a particular application, to which much attention has been given, is medical examination of patients.
For NMR examinations, suitable combinations of magnetic fields are applied to the object to be examined by approriate magnetic coil systems. Excited material within the object is then detected by currents induced in one or more detector coils and are analyzed to provide the required distribution.
Various schemes have been proposed for this purpose. Relevant background references are the disclosures of Edelstein and Hutchison et al in Physics in Medicine & Biology Vol. 25, No. 4, pp. 751-756 (July 1980) and U.S. Pat. Nos. 4,070,611 and 4,431,968 the entirety of each document incorporated herein by reference.
The imaging sequence used for the spin warp NMR imaging proposed by Edelstein et al, shown in FIG. 1, produces spin-echo images with transaxial orientation. The sequence begins when sine-modulated 90.degree. nutation pulse (A) occurs simultaneously with a G.sub.z gradient pulse to produce the spatially selective excitation of nuclear spins. Additional gradient pulses are applied: G.sub.y phase encodes in-plane resolution, G.sub.z phase encodes an image section, and G.sub.x defocuses the free induction decay (FID) to temporally center the later occurring spin-echo signal. A 180.degree. RF pulse (C) occurs simultaneously with a G.sub.z gradient pulse to produce echo refocusing. A read-out gradient G.sub.x frequency encodes (D) the spin-echo signal for resolution along an in-plane axis. The two-dimensional (2D) images were made by iteration of the pulse sequence each time with a different magnitude of G.sub.y at (B).
Each of the above described cycles of the pulse sequence is repeated several times (N) for signal averaging and to cancel FID artifacts which are produced by FID signals active during the spin-echo period but generated by the 90.degree. irradiation of adjacent planar regions outside the selected plane or inner volume chosen for study. To produce the described images, the gradient pulses G.sub.z and G.sub.x are interchanged.
It should be appreciated that the above-described techniques is disadvantageous in various aspects. For example, in the spin warp NMR imaging or Fourier zeugmatography known from U.S. Pat. No. 4,070,611, two phase encoding magnetic gradients must be varied linearly in the respective directions so as to obtain the image information in dependence upon the linear variation between the intensity of the magnetic fields and the corresponding coordinates. However, in practice, the linearity depends on various instrumental parameters, such as the quality of the magnetic field coils, the power supply for the coils, etc. Therefore, in practice, the intensity of the gradient fields is not varied in a linear manner around the origin which is the point in which the gradients contribute to the external static field. This unaviodable non-linearity especially near the zero field origin of the gradient fields causes artifacts in the reconstructed NMR images.